1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a process cartridge that includes a developing unit and is incorporated in an image forming apparatus such as a copier, printer, facsimile machine, and the like.
2. Discussion of the Related Art
Developing units that develop toner images for electrophotographic printing generally employ either a one-component developer or a two-component developer. While the one-component developer includes toner particles only, the two-component developer includes toner particles and magnetic carrier particles. The two-component developer is widely used in a developing unit where the two-component developer is mixed in a developer container so as to frictionally charge the two-component developer (hereinafter “developer”) and cause a developer bearing member to hold the charged developer thereon. Toner particles or toner in the developer carried by the developer bearing member selectively adhere to an electrostatic latent image so that a visible toner image can be formed or developed thereat.
When development is performed, toner is consumed and the consumption of toner decreases toner density in the developer, which can prevent production of high-density images. By contrast, developer having a high toner density can cause background contamination on an image. In other words, to obtain a high quality image, the toner density in the developer contained in the developer container must be controlled so as to remain within a given optimum range.
Some developer units include a toner supply unit to supply toner to the developer container. Such a developing unit includes a toner density detection unit and a toner supply amount controller. The toner density detection unit (hereinafter referred to as a toner density sensor) is a detector or sensor to detect or sense the toner density in the developer container. The toner supply amount controller controls an amount of toner for supplying the developer container. With these units, the supply of toner into the developer container is controlled.
As a toner density sensor, a magnetic sensor is known. The magnetic sensor detects a designated area or a portion of an inner wall of a developer container set as a detection surface to detect changes of magnetic permeability in the developer in the vicinity of the detection surface. The accuracy of this toner density sensor, however, can be degraded by developer accumulating on the detection surface, which can cause the toner supply amount controller to malfunction.
To eliminate the above-described drawbacks, a technique has been proposed in which a planar member is fixedly disposed parallel to a shaft member inside the developer container at a position facing that part of the shaft member of a conveyance screw that conveys developers while agitating the developer inside the developer container which serves as the detection surface, and an elastic sheet is fixedly attached parallel to the planar member. The planar member and the elastic sheet rotate with the conveyance screw, and the elastic sheet scrapes away the developer accumulated on the detection surface of the shaft member of the conveyance screw. By so doing, the developer on the detection surface is agitated and the detection error caused by the accumulation of developer on the detection surface of the toner density sensor can be prevented. It is to be noted that bending rigidity of the planar member is significantly greater than that of the elastic sheet, and therefore deformation of the planar member by removing and agitating the developer repeatedly can be ignored.
However, it can be shown experimentally that, when using the above-described technique with its configuration in which the elastic sheet scrape away developer accumulated on the detection surface, the detected values of the toner density sensor fluctuated in synchronization with a rotation cycle of the elastic sheet as it is scraping away developer accumulated on the detection surface while agitating the developer. This is because the developer density on the detection surface varies before and after the elastic sheet scrapes away the developer on the detection surface. Specifically, before the elastic sheet scrapes away the developer on the detection surface, the elastic sheet pushes the developer onto the detection surface, which increases the developer density on the detection surface. When the elastic sheet cleans the detection surface, the elastic sheet flips up the developer in the vicinity of the detection surface. Therefore, after the elastic sheet scrapes away the developer on the detection surface, there may be a void or space in the vicinity of the detection surface, resulting in a decrease in the developer density on the detection surface.
When the difference in developer densities on the detection surface before and after scraping the detection surface during the agitating operation is large, the detection accuracy of the toner density sensor decreases. Therefore, it is desired to reduce the difference in developer densities on the detection surface. In addition, the difference in developer densities on the detection surface during the agitating operation varies depending on such things as the number of rotations of the conveyance screw, the environment in which the equipment operates, aging of developer, etc. When the fluctuation in developer densities on the detection surface during the agitating operation is large, the toner density detection accuracy also fluctuates depending on use conditions. Therefore, it is desired to reduce differences in developer densities on the detection surface.